The ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.) 52.2 standard rates filters on a MERV (which stands for Minimum Efficiency Reporting Valve) scale which standardizes and simplifies filter efficiency ratings for the public, and provides the initial, as received performance of a filter, allowing a contractor or building owner to select filters based on an efficiency scale. The present ASHRAE 52.2 test does, however, suffer from a number of testing flaws including the deterioration of electret filter performance with time, which is why a new optional testing procedure known as MERVa has been added to the procedure to more adequately reflect the long term performance of a filter. What the new procedure provides is longer initial dosing of KCl particles on the filter prior to the efficiency testing. MERV Designations range from MERV 1 (typically a low efficiency, throwaway filter) up to MERV 20 (a 95%-plus efficiency ULPA filter). The MERV enables one to compare efficiencies of filters at a glance. The higher the MERV rating, the higher the efficiency of the air filter.
In the past furnace filters were made from relatively coarse continuous fibers laid down in a random pattern and built up in layers on a drum and by carding dry staple fibers and forming webs of the carded fibers. Such filters fall short of removing smaller particles and in efficiency performance. Initial efficiency is really not a realistic measure of filter performance because it exaggerates performance for the early part of the actual service life of the filter. This is because when a mechanical air filter is first installed its efficiency is at its lowest point because it has not built up enough lint and particles on the filter to help trap more and smaller lint and particles. Some of these filters had low efficiencies and others had other disadvantages such as low physical integrity and high bulk preventing pleating or making it very difficult.
There are also a host of electret based filters, typically composed of charged polypropylene based fibers, that by MERV indications show good to excellent initial performance. However, these initial readings are misleading. There are two main drawbacks with electret based filter designs. First, the charged nature of the media leads to a rapid drop in efficiency due to the masking of the charges by incoming particles. A minimum is eventually reached whereupon further particle addition leads to a long slow rise in efficiency as the filter fills. The second problem with most electret filters is that they require a metal wire to permanently fold the media. The metal wire, which costs roughly 2 cents/ft2, impacts overall filter system costs. Conventional wisdom is that it is better to use a lower density fiber to construct the filter as more surface area leads to better efficiency performance, but if lower density materials require a metal wire, reconsideration is required.